This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to coatings capable of protecting an underlying coating or substrate from hot corrosion initiated by molten salts.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. Nickel-, cobalt- and iron-base superalloys have found wide use as materials for components of gas turbine engines in various industries, including the aircraft and power generation industries. As operating temperatures increase, the high temperature durability of engine components must correspondingly increase. For this reason, thermal barrier coatings (TBC) are commonly used on components such as combustors, high pressure turbine (HPT) blades and vanes. The thermal insulation of a TBC enables components formed of superalloys and other high temperature materials to survive higher operating temperatures, increases component durability, and improves engine reliability. TBCs typically comprise a thermal-insulating ceramic material deposited on an environmentally-protective bond coat to form what is termed a TBC system. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, a rare-earth metal, and/or another reactive metal), and oxidation-resistant diffusion coatings that may contain compounds such as aluminide intermetallics. Yttria-stabilized zirconia (YSZ) is widely used as the thermal insulating ceramic material of TBC systems because of its high temperature capability, low thermal conductivity, and relative ease of deposition.
In order to achieve higher operating temperatures for gas turbine engines and thus increase their efficiency, alternative materials have been proposed to replace superalloys. In particular, silicon-based non-oxide ceramics, most notably with silicon carbide (SiC), silicon nitride (Si3N4), and/or silicides serving as a reinforcement phase and/or a matrix phase, are candidates for high temperature applications, such as combustor liners, vanes, shrouds, airfoils, and other hot section components of gas turbine engines. However, when exposed in a high-temperature, water vapor-rich combustion atmosphere such as that in a gas turbine engine, components formed of Si-based ceramics lose mass and recede because of the formation of volatile silicon hydroxide (Si(OH)4). The recession rate due to volatilization or corrosion is sufficiently high in a gas turbine engine environment to require an environmentally protective coating, commonly referred to as an environmental barrier coating (EBC).
Critical requirements for an EBC intended to protect gas turbine engine components formed of a Si-based material include stability, low thermal conductivity, a coefficient of thermal expansion (CTE) compatible with that of the Si-based ceramic material, low permeability to oxidants, and chemical compatibility with the Si-based material and a silica scale that forms by oxidation of the Si-based material or (as discussed below) a Si-based protective bondcoat that may be present to promote adhesion of the EBC to the Si-based ceramic material. Silicates, and particularly barium-strontium-aluminosilicates (BSAS; (Ba1xSrx)O—Al2O3—SiO2) and other alkaline-earth aluminosilicates, have been proposed as EBCs in view of their environmental protection properties and low thermal conductivity. For example, U.S. Pat. Nos. 6,254,935, 6,352,790, 6,365,288, 6,387,456, and 6,410,148 to Eaton et al. disclose the use of BSAS and alkaline-earth aluminosilicates as outer protective coatings for Si-based substrates, with stoichiometric BSAS (molar ratio: 0.75BaO.0.25SrO.Al2O3.2SiO2; molar percent: 18.75BaO.6.25SrO.25Al2O3.50SiO2) generally being the preferred alkaline-earth aluminosilicate composition. The use of rare-earth (RE) silicates such as RE2Si2O7 and RE2SiO5 as protective EBC coating materials has also been proposed, as reported in U.S. Pat. Nos. 6,296,941, 6,312,763, 6,645,649, 6,759,151 and 7,595,114. As a particular example, an EBC is disclosed in U.S. Pat. No. 6,759,151 as comprising a rare-earth silicate of formula RE2SiO5, RE4Si3O12, or RE2Si2O7, where RE is Sc, Dy, Ho, Er, Tm, Yb, Lu, Eu, Tb, or combinations thereof. Intermediate layers such as mullite (3Al2O3.2SiO2) and bondcoats such as silicon, disposed between silicon-containing substrates and their protective EBC layers, have been proposed to promote EBC adhesion to the substrate, limit interlayer reactions, and prevent penetration of oxidants into the substrate. For example, commonly-assigned U.S. Pat. Nos. 6,299,988 and 6,630,200 to Wang et al. disclose silicon and silicon-containing materials as suitable bondcoat materials, particularly for substrates containing SiC or silicon nitride. If the particular component will be subjected to surface temperatures in excess of about 2500° F. (about 1370° C.), an EBC can be thermally protected with an overlying thermal barrier coating (TBC) in accordance with commonly-assigned U.S. Pat. No. 5,985,470 to Spitsberg et al. In combination, these layers form what has been referred to as a thermal/environmental barrier coating (T/EBC) system. As noted above, the most commonly used TBC material for gas turbine applications is yttria-stabilized zirconia (YSZ). A transition layer may be provided between a TBC and an underlying EBC, for example, mixtures of YSZ with alumina, mullite, and/or an alkaline-earth metal aluminosilicate as taught in commonly-assigned U.S. Pat. No. 6,444,335 to Wang et al.
It is known that high temperatures within the engine environment can cause contaminants in the fuel to corrode components formed of nickel-, cobalt- and iron-based superalloys, as well as corrode and destabilize TBC systems used to protect them. This phenomenon, known as hot corrosion, is an accelerated corrosion resulting from the presence of impurities such as Na2SO4 and V2O5, which form molten salt deposits on the surface of the component or its protective surface oxides. The corrosive attack by deposits of molten salts can occur over intermediate temperature ranges. For instance, Type I sodium hot corrosion caused by molten Na2SO4 deposits typically occurs over a temperature range of about 880° C. to about 1000° C., whereas vanadium hot corrosion caused by molten V2O5 deposits typically occurs over a temperature range of about 660° C. to about 1000° C. Although the detailed mechanism of hot corrosion varies with the nature of the material attacked and that of the corrodant, in all cases degradation of the structural material or coating can be rapid and the component can be severely damaged in tens to thousands of hours, depending on surface temperature, deposition rate of the molten salt, and concentration of the deleterious cation (for example, sodium and/or vanadium) in the fuel. This type of corrosion, unlike oxidation, is known to consume superalloy components at a rapid rate and lead to catastrophic failure. The susceptibility of Si-based materials such as SiC and Si3N4 by Na2SO4 and V2O5 impurities has been demonstrated in laboratory experiments. On the other hand, the susceptibility of EBC materials, including BSAS and RE silicates, to hot corrosion from Na2SO4 and V2O5 impurities is largely unknown.
Despite the above issues and uncertainties, there is a desire within industries that use gas turbine engines to use cheaper low-grade fuels, which consequently contain higher concentrations of salt impurities and therefore exacerbate the problem of hot corrosion. The most common methods of mitigating hot corrosion include specifying only fuels that contain low and relatively harmless concentrations of corrosive elements (for example, less than 1 ppm Na and less than 0.5 ppm V), water washing fuels to remove soluble Na species prior to combustion, and injecting measured quantities of a suitable compound containing a reactive element, for example, magnesium, to react with vanadium in the fuel and form high melting temperature, relatively inert compounds such as Mg3V2O8. In an era of escalating fuel prices, it would be highly advantageous to run turbine engines on cheaper fuels that contain relatively high levels of corrosive species, for example, in excess of 10 ppm of Na2SO4 and/or V2O5. Aside from the use of reactive elements and washing fuels, additional measures would be desirable to permit the use of such fuels.